Method of multi-level storage in DRAM and apparatus thereof

ABSTRACT

A method of processing data having one of four voltage levels stored in a DRAM cell is comprised of sensing whether or not the data voltage is above or below a voltage level midway between a highest and a lowest of the four levels, setting the voltage on a reference line higher than the lowest and lower than the next highest of the four levels in the event the data voltage is below the midway voltage level, and setting the voltage on the reference line higher than the second highest and lower than the highest of the four levels in the event the data voltage is above the midway point, and sensing whether the data voltage is higher or lower than the reference line, whereby which of the four levels the data occupies is read.

FIELD OF THE INVENTION

This invention relates to dynamic random access memory (DRAM) memories, and in particular to a method of storing a variable level signal in each cell of a DRAM for representing more than one bit in each cell.

BACKGROUND OF THE INVENTION

To store for example two bits in a DRAM cell, it must be able to store four different voltage levels. A problem with such cells, is that noise margins are reduced to one-third that of a one bit per cell DRAM, which is too low to withstand the occasional α-particle bit.

A second problem with multi-bit storage cells relates to the method of sensing. No simple method of sensing has previously been designed, although attempts have been made to solve this problem, e.g. as described in the publication by M. Aoki, et al, “A 16-Levels/Cell Dynamic Memory”, ISSCC Dig. TECH. Papers 1985, pp. 246-247, and in T. Furuyama et al, “An Experimental Two-Bit/Cell Storage DRAM for Macrocell or Memory-On-Application”, IEEE Journal of Solid State Circuits, Vol. 24, No. 2, pp. 388-393, April 1989. The technique described by Aoki cannot use normal sense amplifiers. It requires a precision analog D to A converter to implement a staircase waveform and a charge amplifier to sense data. The technique described by Furuyama requires the generation of precision reference levels to distinguish between four levels. These levels are not self-compensated for offsets developed in the sensing operation, and this method suffers from poor signal margin. Hidaka et al describe a technique for simultaneously reading two cells at a time in the article “A divided/Shared Bitline Sensing Scheme for 64Mb DRAM Core” in the 1990 Symposium on VLSI Circuitry 1990, IEEE, p,. 15, 16 which while describing dividing a bitline, is not related to multiple bit storage in a single cell.

DRAMs have previously been built with cells holding up to sixteen bits of storage, e.g. in the aforenoted article by M. Aoki et al, for use in file memories. A 4 K test array is believed to have been the largest memory built using this design. Leakage characteristics of the DRAM cell were required to be very tightly controlled and even then, accurate sensing of the small voltage differences between levels becomes very difficult. Another problem with this scheme was the length of time required to access: a single read cycle required 16 clocks for the read followed by 16 clocks for the restore.

To implement a 2 bit DRAM, one can define the cell as storing one of four voltage levels V_(cell0), V_(cell1), V_(cell2) and V_(cell3), and reference voltage midpoints between these four voltage, which can be defined as V_(ref1), V_(ref2) and V_(ref3). These midpoints can be referred to, to differentiate between the four voltage levels. The relative voltage of these levels are shown in Table 1 below.

STORAGE REFERENCE ACTUAL VOLTAGES VOLTAGES VOLTAGE V_(cell1) V_(DD) V_(ref3) 5/6 V_(DD) V_(cell2) 2/3 V_(DD) V_(ref2) 1/2 V_(DD) V_(cell1) 1/3 V_(DD) V_(ref1) 1/6 V_(DD) V_(cell0) V_(SS)

The storage voltages are the actual voltages stored in the cells, although the sensing voltages are somewhat more attenuated. Since sensing takes place on the bitlines which divide cell charge by the cell to bitline capacitance ratio, much lower voltages than those in the cell are actually sensed. In a standard DRAM, these voltage differences are in the order of 100-300 mV. It is the voltage midpoints between these smaller signals that must finally be generated to allow for correct sensing.

Furuyama et al in the article noted above describes one method of sensing these voltages. Furuyama et al used three sense amplifiers and three approximate midpoint sensing voltages. The cell charge is shared with the bitline, the bitline is split into three sections (sub-bitlines) and three sense amplifiers determine whether the cell charge is above or below their particular reference voltages. This data is then converted to two bits and a resulting output. Reconversion of the two bits allows approximate values to be driven into the bitlines so that restore takes place after the read cycle. A write cycle operates in the same way as the restore section of the read cycle.

It should be noted that since the cell shares charge with three sub-bitlines, and the reference cell with only one sub-bitline, the reference voltage is about three times larger than it should be for sensing, casting doubt on the operability of this design. Secondly, three sense amplifiers are used, and since sense amplifiers have been growing proportionally larger and larger with each generation of memory, a minimum of sense amplifiers is desirable. A third problem is that the reference voltage is not stored on a cell whose leakage does not characteristics track the leakage of the data cells, introducing another source of error into the circuit.

SUMMARY OF THE PRESENT INVENTION

In the present invention a method and circuit has been designed which substantially solves some of the above-identified problems. Only two sense amplifiers are required, which generate the sensing voltages at the time of sensing . In the present invention each bitline is split exactly in half, rather than into thirds, by use of a switch. The noise margins are relatively large, equivalent to that of a standard DRAM maintaining reliability, and the present design can be used as a standard one bit per cell DRAM as an alternative to a multiple bit per cell DRAM, which increases its universality, allows it to be used in present designs, and increases yield.

In accordance with an embodiment of the present invention, a method of processing data having one of four levels stored in a DRAM cell is comprised of sensing whether or not the data voltage is above or below a voltage level midway between a highest and a lowest of the four levels, setting the voltage on a reference line higher than the lowest and lower than the next highest of the four levels in the event the data voltage is below the midway voltage level, setting the voltage on the reference line higher than the second highest and lower than the highest of the four levels in the event the data voltage is above the midway point, and sensing whether the data voltage is higher or lower in voltage than the reference line, whereby which of the four levels the data bit occupies is read.

In accordance with an embodiment of the present invention, a method of processing data having one of plural levels stored in a DRAM cell capacitor is comprised of dumping the charge of the cell capacitor on a first of a pair of conductors of a folded bitline, maintaining the other of the pair of conductors split into other sub-bitline conductors and charging each of the other sub-bitline conductors to an intermediate voltage, splitting the first of the conductors into first sub-bitline conductors, sensing the sub-bitlines to determine whether the charge of the cell has a higher voltage than the intermediate voltage of one of the other sub-bitline conductors and providing a logic level result signal, storing the logic level result signal in a dummy cell capacitor, setting a charge storage capacitor and all of the sub-bitlines other than the first sub-bitline maintaining a voltage resulting from the dumped charge to a predetermined voltage, dumping charge stored in the dummy cell capacitor on the sub-bitlines and charge storage capacitor, thereby varying the predetermined voltage stored thereon to a degree related to the capacities of the dummy cell capacitor, the charge storage cells and the predetermined voltage, to a level above or below the intermediate level, isolating the sub-bitlines, applying the intermediate voltage to one of the other sub-bitline conductors, comparing the cell voltage on one sub-bitline with the voltage on the other sub-bitline carrying the level above or below the intermediate level to obtain a first logic bit, and comparing the voltages carried by the other sub-bitlines to obtain a second logic bit, whereby the first and second logic bits are indicative of one of four logic states corresponding to one of the plural levels stored in the DRAM cell.

BRIEF INTRODUCTION TO THE DRAWINGS

A better understanding of the invention will be obtained by reference to the detailed description below, in conjunction with the following drawings, in which:

FIG. 1 is a diagram illustrating various voltage levels referred to in the description,

FIG. 2 is a block diagram used to illustrate the basic concepts of the invention;

FIG. 3 illustrates a block diagram in six steps of a read cycle,

FIG. 4 illustrates in block diagram two steps of a write or restore cycle,

FIG. 5 is a schematic diagram illustrating an embodiment of the invention, and

FIG. 6 illustrates a timing diagram of the schematic illustrated in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

For a DRAM cell to store two bits using a single cell capacitor, the cell capacitor should store one of four voltage values V_(cell0), V_(cell1), V_(cell2) or V_(cell3), wherein V_(cell0) represents the lowest and V_(cell3) represents the highest cell voltage. To differentiate between the voltages, mid-point voltages V_(ref1)V_(ref2) and V_(ref3) are defined, as shown in FIG. 1. It may be seen that if the lowest actual cell voltage V_(cell0) is V_(SS) or zero, V_(ref1) is one-sixth the highest voltage V_(DD), V_(cell1) is one-third V_(DD), V_(ref2) is one-half V_(DD), V_(cell2) is two-thirds V_(DD), V_(ref3) is five-sixths V_(DD) and V_(cell3) equals V_(DD). Thus it may be sen that V_(ref1) is midway between V_(cell0) and V_(cell1), V_(ref2) is midway between V_(cell0) and V_(cell3) and V_(ref3) is midway between V_(cell2) and V_(cell3).

FIG. 2 will be used to illustrate the basic concept of the invention. A sense amplifier 1 can be connected to a pair of conductors 3A and 3B which form a folded bitline. Another sense amplifier 5 can be connected to a pair of conductors 7A and 7B which form the remainder of the folded bitline. In practice conductor 7A is a continuation of conductor 3A, and conductor 7B is a continuation of conductor 3B. The bitline is shown split in half as shown by dotted line 9. In practice, however, any of the sub-bitline conductors may be connected to any others, e.g. via FET switches.

Capacitor 11 represents a cell on which charge is stored in one of four voltage levels. It is desired to read the level and output two binary bits representing the charge level stored on capacitor 11.

The detailed sequence will be described below. It should be noted that sub-bitline conductor 7A can be brought to V_(ref2), which is one-half V_(DD). The voltage on sub-bitline 3B resulting from the charge stored on cell capacitor 11 is then compared with the voltage on conductor 7A to determine whether it is above or below V_(ref2).

If the voltage on conductor 3B is above V_(ref2), then the continuous conductor 3A-7B is brought to V_(ref3), Which is midway between the voltage level V_(cell2) and V_(cell3). If the voltage on conductor 3B is below V_(ref2), then the voltage on continuous conductor 3A, 7B is brought to V_(ref1), which is midway between the V_(cell1) and V_(cell0) voltages. The continuous conductor 3A, 7B is referred to herein as a reference line.

It may be seen, therefore, that the voltage on the reference line is either above or below one-half V_(DD), i.e. V_(ref2), and is established midway between the only two voltages Which conductor 3B can have, V_(cell0) and V_(cell1), or V_(cell2) and V_(cell3).

The voltage on sub-bitline 3B is then compared with the voltage on the reference line to determine whether it is above or below that voltage. If it is above that voltage the logic voltage must be either V_(cell1) or V_(cell3); whichever one it is, was established by the first determination of whether the voltage on conductor 3B was above or below the midway voltage V_(ref2). Similarly if the voltage on conductor 3B is below the voltage on the reference line, the logic output represents either V_(cell0) or V_(cell2), and again whichever one it is, was previously determined by the original determination of whether the voltage on conductor 3B is below or above V_(ref2). In practice, the voltage on conductor 3B could be compared again With V_(ref2), which is on lead 7A, to select which of either of the two cell voltage possibilities should be selected.

The result is a two bit binary bit word representing which of the four charge levels is stored in capacitor 11.

FIG. 3 illustrates in more detail a sequence of the steps in the process. Consider first bitlines BL and BL*. These bitline references are not shown as such, but their connection points to a pair of sense amplifiers 13 and 15 are shown referenced BL and BL*. Sense amplifier 13 is provided to sense bit 0 and sense amplifier 15 is provided to sense bit 1. It is important that bitlines should be able to be split exactly in half, e.g. by apparatus such as a switch into sub-bitline conductors BLA, BLB, BL*A and BL*B. The sense amplifiers can be enabled or disabled as required. Prior to step 1 all bitline segments are charged to the voltage V_(ref2) which is ½ V_(bb)V_(dd).

In step 1, sub-bitlines BL_(A) and BL_(B) are disconnected from the V_(ref2) reference voltage, connected together and the sense amplifiers are not connected to the bitline. The cell capacitor 11 then dumps its charge onto the BL line formed of conductors BL_(A) and BL_(B), resulting in a voltage which for example is $\frac{\left\lbrack V_{DD} \right.}{2} + {\frac{C_{s}^{*}\left( {V_{cell2} - V_{REF2}} \right)}{C_{s} + C_{BL}}\quad {i.e.\quad \frac{V_{DD}}{2}}} + \frac{C_{s}}{C_{s} + C_{BL}} + {\frac{2V_{DD}}{\left. 3 \right\rbrack}.\quad}$ $\frac{V_{DD}}{2} + \frac{C_{s}^{*}\left( {V_{cell2} - V_{REF2}} \right)}{C_{s} + C_{BL}}$

where C_(S) is the cell capacitance and C_(BL) is the capacitance of the entire bitline.

The voltage V_(ref2) which is ½ V_(DD) is applied to the BL* conductors BL*_(A) and BL*_(B).

In step 2, the two halves of the BL conductor are separated, and the bit 1 sense amplifier is connected to sense the voltage on conductor BL_(B) to determine whether it is above or below the voltage V_(ref2) which is on the BL*_(B) conductor. Since the voltage is V_(cell2) which is above V_(ref2), a logic level one signal is stored in dummy cell 17 which is connected to the BL_(B) lead.

In step 3 the sense amplifier 15 and dummy cell 17 are disconnected from the bitline, and charge storage Ccell capacitor 18 is connected to lead BL_(B). Conductors BL_(B), BL*_(B), and BL*_(A) are all connected together, and the midpoint voltage V_(ref2) is applied thereto.

In step 4 the logic level signal stored in dummy cell 17 is dumped to the sub-bitlines BL_(B), BL*_(B) and BL*_(A). The charge is also shared with Ccell 18, which has one half the capacitance of a normal cell. This charge sharing on the three half bitlines plug Ccell creates the exact reference level needed for the 2nd phase of sensing.

The total capacity of the Ccell 18 should be established so that the resulting voltage on the sub-bitlines is, in this example, V_(ref3). Thus for example if the voltage on the Ccell and sub-bitlines was established at V_(ref2), one-half V_(DD) in step 3, with the charge on dummy cell 17 having been established with full logic level V_(DD) in step 2, when it is connected to the combined sub-bitlines, in step 4, its charge, being shared with the Ccell, should result in a voltage. ${\left( \frac{V_{DD}}{3} \right)\frac{C_{s}}{C_{s} + C_{BL}}} + \frac{V_{DD}}{2}$

i.e. V_(ref3), which is midway between V_(cell2) and V_(cell3).

On the other hand, if in step 2 the sensed bit was a zero, charge on dummy cell 17 would have been zero or V_(SS). When connected to the combined bitlines in step 4 it would receive charge from the Ccell 18, causing a reduction in voltage to V_(ref1), which is midway between V_(cell1) and V_(cell0). Thus it may be seen that the combined sub-bitlines form a reference line, the voltage of which can be compared with that on cell 11, and corresponds to reference line 3A, 7B described with reference to FIG. 2.

In step 5 the Ccells 18 and dummy cell 17 are disconnected and each of the sub-bitlines are isolated. The voltage V_(ref2) of one-half V_(DD) is applied to the sub-bitline BL*_(B). It may be seen that the sub-bitline BL*_(B) is now at the midpoint V_(ref2), both sub-bitlines BL*_(A) and BL_(B) are at the reference line voltage V_(ref3) (or V_(ref1) if the original cell voltage had been below V_(ref2)), and the sub-bitline BLA is at the cell capacitor 11 voltage.

The sense amplifiers 13 and 15 are then connected to their respective associated sub-bitlines. Bit zero from sub-bitlines BL*_(A) and BL_(A) is sensed in sense amplifier 13, and preferably bit 1 from sub-bitline BL*_(B) and BL_(B) is resensed. The outputs of the sense amplifiers 13 and 15 form a two bit binary word (bit 0 and bit 1) representing the level originally stored on cell 11.

FIG. 4 illustrates in steps 7 and 8 a write or restore operation.

Either immediately after step 6, for the restore operation, or at the beginning of a write operation, the sub-bitlines are separated and the sense amplifiers are disabled. In the case of a restore operation, the logic levels are already present on the sub-bitline conductors. In the case of a write operation, binary bits are written to each of the sub-bitline conductors, or to as many as are required to determine the level of the bit to be stored. To restore V_(cell0) or V_(cell3) the full logic level is left in the cell. To restore V_(cell1) or V_(cell2) the full logic level must be attenuated by ⅓ as shown in the example step 8. In step 8, the required sub-bitlines BL*_(A), BL_(A) and BL_(B) shown are short-circuited together and the charge thereon is shared. This shared charge is written to cell capacitor 11 by connection of capacitor 11 thereto.

It should be noted that the concept described above has certain very significant advantages. For example no changes are required to either the currently used DRAM basic memory cell or to the DRAM manufacturing process.

Another advantage of this invention is that the first sensed step, i.e. step 2 of the read cycle described with reference to FIG. 3, can be simplified to appear identical to a standard one-bit-per-cell sense. If only the values 1,1 and 0,0 are stored in the cell, then the first sense has nose margins equal to V_(DD)/2, the same as a standard DRAM. Indeed, one step regenerative sensing is possible by allowing the sense amplifier to be enabled earlier in the operation and by not bothering to split the bitlines. Therefore if two bits per cell in the present design is not used, the memory wafers can still be used as standard one bit per cell structure. The resulting overhead to use the present invention is the extra sense amplifier, bitline splitting switches and cycle control logic. However the same design can be used for either one or two bits per cell application.

Reference is now made to FIGS. 5 and 6. In FIG. 5, a schematic diagram of an embodiment of the present invention is shown. The convention is used of the bitline conductors referenced in FIG. 4, that is BL_(A), BL*_(A), BL_(B) and BL*_(B). All of the transistors used in this embodiment are N channel field effect transistors (FETs). While steps 1-6 are described in detail, a person understanding the description below will be able to understand how the restore and write operations proceed without further explanation.

Conductors BL_(A) and BL_(B) and BL*_(A) and BL*_(B) are connected to respective source and drains of FET transistors 20 and 21 respectively, whose gates are driven by timing signals CB_(L) and CB_(L)* respectively. Bitline conductor BL_(B) is connected to a terminal of sense amplifier 22 via the source-drain circuit of FET transistor 23, while bitline conductor BL_(A) is connected to sense amplifier 24 Via FET 25. Similarly bitline conductor BL*_(B) is connected to the other terminal of sense amplifier 22 via FET 26 and bitline conductor BL*_(A) is connected to the other terminal of sense amplifier 24 via FET 27. FETs 23 and 26 are operated via a timing signal ISO2 which is applied to their gates, and FETs 25 and 27 are enabled by timing signal ISO1 applied to their gates.

Bitline precharge voltage V_(BLP) is applied to bitlines BL*_(A) and BL_(A) via FETs 28 and 29, and to bitline conductors BL*_(B) and BL_(B) via FETs 30 and 31.

The charge to be sensed is stored on cell capacitor 32, which is connected to bitline BL_(A) via FET 33, which is driven by the timing signal WL1 received from a word line applied to its gate.

In operation, initially the bitline portions are isolated from each other by the C_(BL) and C_(BL*) timing voltage being low rendering FETs 20 and 21 non-conductive, and precharge voltage is applied to the four bitline conductors via transistors 28, 29, 30 and 31 due to timing voltages M_(A1), M_(A1*), M_(A2) and M_(A2*) being high. At the same time the bitline conductor voltages are equalized via FETs 39 and 34 short-circuiting bitline conductor pairs BL_(A) and BL_(*A), and BL_(B) and BL_(*B) respectively, FET 39 being enabled by the E_(Q1) timing voltage being applied to its gate, FET 34 being enable by E_(Q2).

Once precharge has been completed, the timing voltages E_(Q1), E_(Q2) and M_(A1), M_(A2) and M_(A2*) go low, causing transistors 39, 34, 29, 31 and 30 to open. Timing voltage M_(A1*) remains high, maintaining precharge voltage (V_(ref2) in step 1 of FIG. 3) on bitline conductor B_(L*A).

The next step is for the timing voltage C_(BL) to go high for a short interval and at the same time for W_(L1) to go high. This causes FET 20 to conduct, connecting bitline conductors BL_(A) and BL_(B) together, and at the same time transistor 33 conducts, causing the charge from bit storage capacitor 32 to be dumped to the bitline conductor BL_(A). Since the timing voltages ISO1 and ISO2 are low, the transistors 25, 27, 23 and 26 are open, isolating the sense amplifiers 24 and 22 from the bitlines. The stage of step 1 in which the cell charge from capacitor 32 is dumped onto the bitline conductors BL_(A) and BL_(B) and that the remaining bitline conductors BL*_(A) and BL_(*B) have been precharged to a midpoint reference voltage V_(BLP)(V_(ref2)) has thus been completed.

Once the charge has been dumped onto the bitline, the C_(BL) timing voltage returns to a low level, isolating the bitline conductors BL_(A) and BL_(B) and following this the ISO2 voltage goes high, enabling transistors 23 and 26. The timing voltages V_(S2) and V_(R2) flip, causing sense amplifier 22 to sense the bit stored on bitline conductor B_(LB) relative to the midpoint reference voltage stored on bitline B_(L*B). The full logic level value of the sensed bit (0 or 1) is then applied by sense amplifier 22 to the bitline. Timing voltage W_(L2) going high enables FET 35, causing the sensed bit logic level voltage to be stored in dummy capacitor cell 36.

The timing voltage W_(L2) then drops, isolating capacitor 36. The voltages V_(S1) and V_(S2) applied to sense amplifier 22 reverse, disabling sense amplifier 22. This completes step 2, wherein the bit has been sensed and stored in the dummy cell capacitor 36.

The timing voltage ISO2 then drops, causing transistors 23 and 26 to isolate the bitline conductors BL_(B) and BL_(*B) from sense amplifier 22. The timing voltages EQ2, M_(A2) and M_(A2*) then go high, causing transistor 34 to conduct and short-circuiting bitline conductors BL_(B) and BL_(*B), and causing FETs 31 and 30 to conduct, allowing reference voltage V_(BLP)(V_(ref2)) to be reapplied to the bitline conductors BL_(B) and BL_(*B). Then the timing voltage C_(BL*) goes high, causing transistor 21 to conduct, joining bitline conductors BL_(*A) with BL_(*B) and BL_(B). Accordingly the reference voltage V_(BLP) is applied to those three bitline conductors, which are equalized. Ccell capacitor 37 is then connected to the bitline conductor BL_(B) via FET 38 due to the gate of FET 38 going high with the timing voltage V_(CL). This completes operation through to the completion of step 3 described with reference to FIG. 3.

The timing voltage M_(A2) and M_(A2*), as well as M_(A1*) then go to low level, inhibiting FETs 31, 30 and 28, cutting off reference voltage V_(BLP) from the bitline conductors.

The next step is for the timing voltage WL2 to go high. This causes the charge stored on dummy cell capacitor 36 to be dumped onto the three interconnected bitline conductors BL_(B), BL_(*B) and BL_(A), and as well onto Ccell 37. This completes step 4 described with reference to FIG. 3.

The timing voltage EQ2 then drops to low level, removing the short circuit between the bitline conductors BL_(B) and BL_(*B), and the timing voltage C_(BL*) drops to low level, causing separation of the bitline conductors BL_(*A) and BL_(*B). The four bitline conductors are thus mutually isolated.

The timing voltage M_(A2*) then goes to high level for a short period, recharging the bitline conductor B_(L*B) to the reference voltage V_(BLP). The result, at this stage, is that the bitline conductor B_(L*B) is at the voltage of reference level V_(BLP), the bitline conductors B_(L*A) and BL_(B) are charged to the distributed level resulting from the charge previously stored on dummy capacitor 36, and the bitline conductor B_(LA) is charged to the level stored on the bit storage cell capacitor 32. The completes step 5 described with reference to FIG. 3.

The timing voltages ISO1 and ISO2 then go to high level, enabling FETs 23 and 26, and 25 and 27, thus connecting sense amplifiers 22 and 24 to the bitlines. The timing voltages V_(S1) and V_(S2) and V_(R1) and V_(R2) are inverted, causing operation of sense amplifiers 22 and 24, thus sensing the bit stored on the two bitlines BL_(A) and BL_(*B) relative to the voltages (which are at the same voltage level) on bitline conductors BL_(*A) and BL_(B). This completes the operation of step 6 described with reference to FIG. 3.

The output result of sense amplifier 22 and 24 are thus two bits which describe the charge level stored in capacitor 32 to the accuracy of 2²=4 levels, as described above.

It should be noted that there are several ways of expanding the above invention so that more than four charge levels stored on bit storage capacitor 32 can be detected. One way is to use a variable reference voltage V_(BLA) which is changed in the direction of the sensed bit level following either a first or successive sensing steps. A second way is to use more than the three voltage reference levels ⅙ V_(DD), ½ V_(DD), and ⅚ V_(DD) described. The bitlines may be divided into three sections for three successive sensing operations to get 8 levels, 4 sections or 16 levels, etc. By successive sensing and charge juggling between the dummy capacitor and Ccell capacitors, first coarse and then finely tuned, voltage references can be established, following which the sensing of the charge in the memory cell can be effected as being either above or below the established voltage reference.

A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above. All of those which fall within the scope of the claims appended hereto are considered to be part of the present invention. 

I claim:
 1. A method of processing data having one of four voltage levels stored in a DRAM cell comprising: (a) dumping a charge stored in the DRAM cell onto sub-bitlines of a pair of bitlines to provide a sensing voltage of one of four sensing voltage levels on the sub-bitlines; (a b) sensing whether or not the data the sensing voltage is above or below a voltage level midway between a highest and a lowest of said four sensing voltage levels, (b c) setting the voltage, on a reference line sub-bitline of the pair of bit lines, higher than the lowest and lower than the next highest of said four sensing voltage levels in the event the data sensing voltage is below said midway voltage level, and setting the voltage on the reference line sub-bitline higher than the second highest and lower than the highest of said four sensing voltage levels in the event the data sensing voltage is above said midway voltage level, and (c d) sensing whether the data sensing voltage is higher or lower than the voltage on the reference line sub-bitline, whereby which of the four levels the data occupies is read.
 2. A method as defined in claim 1 in which the voltage on the reference line sub-bitline is set at approximately one half the voltage difference between either of the lowest or highest sensing voltage level and the adjacent one of the four sensing voltage levels.
 3. A method as defined in claim 1 in which said four sensing voltage levels are at 0, ⅓, ⅔ and 1 times a power supply voltage scaled by the cell to bitline capacitance ratio, and in which the voltage on the reference line in step (b) sub-bitline in step (c) is set at either ⅙ or ⅚ the power supply voltage in step (b) scaled by the cell to bitline capacitance ratio.
 4. A method as defined in claim 1 including charging a dummy capacitor to a level representing whether or not said data is above or below said midway voltage, and setting the voltage on the reference line sub-bitline by establishing a voltage level thereon which is midway between the highest and lowest of said four sensing voltage levels, then raising or lowering the voltage level thereon by dumping the charge from said dummy capacitor thereon.
 5. A method of processing data having one of plural levels stored in a DRAM cell capacitor comprising: (a) dumping the charge of the cell capacitor on a first conductor of a pair of conductors of a folded bitline, (b) maintaining the other conductor of said pair of conductors split into other sub-bitline conductors and charging each of said other sub-bitline conductors to an intermediate voltage, (c) splitting said first of said pair of conductors into first sub-bitline conductors, (d) sensing one of said sub-bitline conductors to determine whether the charge of said cell has a higher voltage than the intermediate voltage of one of said other sub-line conductors and providing a logic level result signal, (e) storing said logic level result signal in a dummy cell capacitor, (f) setting a charge storage capacitor and all of the sub-bitlines, other than a first sub-bitline conductor on which the charges charge of the cell capacitor was dumped, at to a predetermined voltage, (g) dumping charge stored in the dummy cell capacitor on said sub-bitlines sub-bitline conductors other than the first sub-bitline conductor to which the charge of the cell capacitor was dumped, and on said charge storage capacitor together, thereby varying the predetermined voltage stored thereon to a degree related to the capacitance of said dummy cell capacitor, said charge storage capacitor and said predetermined voltage, to a level above or below the intermediate level, (h) isolating the sub-bitlines, (i) applying said intermediate voltage to one of said other sub-bitline conductors, (j) comparing the cell voltage on one sub-bitline with the voltage on said other sub-bitline carrying said level above or below the intermediate level to obtain a first logic bit, and comparing the voltages carried by the other sub-bitlines to obtain a second logic bit, whereby said first and second logic bits are indicative of one of four states corresponding to one of said plural levels stored in the DRAM cell.
 6. A method as defined in claim 5 in which said intermediate and said predetermined voltages are the same.
 7. A method as defined in claim 6 in which said intermediate and predetermined voltages are the same midpoint voltage between a highest and lowest voltage state representative of four logical states.
 8. A method as defined in claim 3, including the further steps of short circuiting two or three of the sub-bitlines to share charge thereon and establish a common voltage level, and storing said shared charge corresponding to the common voltage level on said cell, whereby a restore or write operation results.
 9. A method as defined in claim 7, including the further steps of writing logic voltage levels to each of the sub-bitline conductors, short circuiting two or three of the sub-bitlines to share charge thereon and establish a common voltage level, and storing said shared charge corresponding to the common voltage level on said cell, whereby a write operation results.
 10. A method as claimed in claim 1 wherein the voltage on the reference sub-bitline is set by charging a capacitor to a level depending on the results of step (b) and discharging the capacitor onto the reference sub-bitline.
 11. A method as claimed in claim 10 wherein the capacitor is discharged onto three sub-bitlines including the reference sub-bitline.
 12. A method as claimed in claim 1 including charging a capacitor to a level representing whether or not said sensing voltage is above or below said midway voltage, setting the voltage on the reference sub-bitline by establishing a voltage level thereon which is midway between the highest and lowest of said four sensing voltage levels and then raising or lowering the voltage level thereon by dumping the charge from said capacitor thereon.
 13. A method as claimed in claim 1 wherein the sensing steps (b) and (d) are by sensing amplifiers coupled to opposite ends of the pair of bitlines.
 14. A method as claimed in claim 13 wherein the sense amplifiers are disconnected from the pair of bitlines during steps (a) and (c).
 15. A method of reading data having one of plural voltage levels stored in a DRAM cell comprising: dumping a stored voltage from the DRAM cell to sub-bitlines which are disconnected from sense amplifiers; in a first sense amplifier, sensing the dumped stored voltage on a first sub-bitline relative to a first reference voltage to define a first bit of a multibit representation of the stored data; dumping a voltage, the level of which is dependent on the value of the first bit, onto a second sub-bitline disconnected from the sense amplifiers to provide a second reference voltage; and in a second sense amplifier, sensing the dumped stored voltage on a third sub-bitline relative to the second reference voltage to define a second bit of said multibit representation of the stored data.
 16. A method as claimed in claim 15 wherein one of four voltage levels is stored in the DRAM cell and each of a pair of bitlines is formed of two sub-bitlines.
 17. A method as claimed in claim 16 wherein the voltage, the level of which is dependent on the value of the first bit, is dumped onto three sub-bitlines of the pair of bitlines.
 18. A method as claimed in claim 17 in which said four levels are at 0, {fraction (1/3)}, {fraction (2/3)} and 1 times a power supply voltage scaled by the cell to bitline capacitance ratio, and in which the second reference voltage is set at either {fraction (1/6)} or {fraction (5/6)} the power supply voltage scaled by the cell to bitline capacitance ratio.
 19. A method as claimed in claim 15 wherein a sense amplifier is coupled at each end of a pair of bitlines which are divided into sub-bitlines.
 20. A method as claimed in claim 15 wherein the step of dumping a voltage to provide a second reference voltage comprises charging a capacitor to a level representing the first bit, setting the voltage on sub-bitlines by establishing a voltage level thereon which is midway between the highest and lowest of four levels and then raising or lowering the voltage level on the sub-bitline by dumping the charge from said capacitor thereon.
 21. A method as claimed in claim 15 wherein the stored voltage from the DRAM cell is dumped onto sub-bitlines of a single pair of bitlines with a sense amplifier coupled to each end thereof.
 22. A method of processing data having one of plural voltage levels stored in a DRAM cell comprising: dumping a charged stored in the DRAM cell onto sub-bitlines of a pair of bitlines to provide a sensing voltage of one of plural sensing voltage levels on the sub-bitline; sensing whether the sensing voltage is above or below a voltage level midway between a highest and a lowest of said plural levels, setting the voltage, on a reference sub-bitline of the pair of bitlines, to a level which is dependent on whether the sensing voltage is sensed to be above or below the midway voltage level; and sensing whether the sensing voltage is higher or lower than the voltage on the reference sub-bitline.
 23. A method as claimed in claim 22 wherein the voltage on the reference sub-bitline is set by charging a capacitor to a level which depends on whether the sensing voltage is above or below said voltage level midway between the highest and lowest of said plural levels, and discharging the capacitor onto the reference sub-bitline.
 24. A method as claimed in claim 23 wherein the capacitor is discharged onto three sub-bitlines including the reference sub-bitline.
 25. A method as claimed in claim 22 wherein the sensing steps are by sensing amplifiers coupled to opposite ends of the pair of bitlines.
 26. A method as claimed in claim 25 wherein the sense amplifiers are disconnected from the pair of bitlines during the steps of dumping a charge and setting the voltage.
 27. A method as claimed in claim 22 wherein one of four voltage levels is stored in the DRAM cell.
 28. A method as claimed in claim 27 wherein the step of setting the voltage includes charging a capacitor to a level representing whether or not said sensing voltage is above or below said midway voltage, setting the voltage on the reference sub-bitline by establishing a voltage level thereon which is midway between the highest and lowest of said plural levels, and then raising or lowering the voltage level thereon by dumping the charge from said capacitor thereon.
 29. A method of processing data having one of plural voltage levels stored in a DRAM cell comprising: dumping the voltage stored in said DRAM cell onto sub-bitlines of a pair of bitlines to provide a sensing voltage; sensing the sensing voltage on one of the sub-bitlines relative to a first reference level to provide a first bit; dumping a charge, the level of which depends on the value of the first bit, onto a sub-bitline to establish a second reference level; and sensing whether the sensing voltage on another of the sub-bitlines is above or below the second reference level to provide a second bit.
 30. A method as claimed in claim 29 wherein the sensing steps are by sensing amplifiers coupled to opposite ends of the pair of bitlines.
 31. A method as claimed in claim 30 wherein the sense amplifiers are disconnected from the pair of bitlines during the steps of dumping.
 32. A method as claimed in claim 29 wherein one of four voltage levels is stored in the DRAM cell and each of a pair of bitlines is formed of two sub-bitlines.
 33. A method as claimed in claim 32 wherein the charge which depends on the value of the first bit is dumped onto three sub-bitlines of the pair of bitlines.
 34. In a DRAM having plural cell capacitors coupled to a pair of conductors of a folded bitline, a method of processing data having one of plural levels stored in one of said cell capacitors comprising: dumping the charge of said one of said cell capacitors on a first conductor of the pair of conductors of the folded bitline; splitting said first of said pair of conductors into first sub-bitline conductors; charging a sub-bitline of a second conductor of said pair of conductors to an intermediate voltage; sensing one of said first sub-bitline conductors to determine whether the dumped charge has a higher voltage than the intermediate voltage on said sub-bitline of the second conductor and providing a first logic level result signal; storing said first logic level result signal in one of said cell capacitors; dumping the stored first logic level result signal onto sub-bitline conductors to vary the voltage stored thereon to a degree relating to the capacities of the cell capacitor from which the second first logic level result signal is being dumped and relating to the sub-bitlines, to a level above or below the intermediate voltage; and comparing the cell voltage on another of the first sub-bitline conductors against the level above or below the intermediate voltage to obtain a second logic level result signal.
 35. A method of processing data having one of plural voltage levels stored in a DRAM cell comprising: providing a pair of bitlines having a sense amplifier connected at each end thereof, the bitlines being formed of sub-bitlines, the DRAM cell being connected to only one of the sub-bitlines; with the sense amplifiers disconnected from the bitlines, dumping the voltage stored in the DRAM cell onto two sub-bitlines of the pair of bitlines to provide a sensing voltage level; with one of the sensing amplifiers, sensing the sensing voltage on one of the sub-bitlines relative to a first reference level on another sub-bitline to provide a first bit; with the sense amplifiers disconnected from the bitlines, dumping a charge which depends on the value of the first bit onto three sub-bitlines of the pair of bitlines to establish a second reference level; and with the other of the sensing amplifiers, sensing whether the sensing voltage on another of the sub-bitlines is above or below the second reference level to provide a second bit.
 36. A dynamic random access memory comprising: an array of DRAM cells, each comprising a storage capacitor and a transistor for charging the capacitor from or discharging the capacitor to a single bitline of a pair of bitlines, and a plurality of word lines for enabling the transistors of the DRAM cells; means for sensing whether a sensing voltage on a bitline, discharged from the data voltage stored on a DRAM cell capacitor, is above or below a first reference level; means for setting the voltage on a reference line to a level which is dependent on whether the sensing voltage is sensed to be above or below the first reference level; and means for sensing whether the sensing voltage is higher or lower than a second reference level.
 37. A dynamic random access memory as claimed in claim 36 wherein pairs of bitlines are coupled to sense amplifiers at opposite ends thereof and the bitlines are divided into sub-bitlines.
 38. A dynamic random access memory as claimed in claim 37 wherein each bitline is divided into two sub-bitlines.
 39. A dynamic random access memory comprising: means for discharging a stored voltage from a DRAM cell to a single bitline of a pair of bitlines; means for splitting the bitlines into sub-bitlines; means for comparing the voltage on a sub-bitline to a first reference voltage to define a first bit of a multibit representation of a plural voltage level; means for setting a second reference voltage, the level of which is dependent on the value of the first bit; and means for comparing the voltage on a second sub-bitline to the second reference voltage to define a second bit of said multibit representation of the plural voltage levels.
 40. A dynamic random access memory as claimed in claim 39 wherein pairs of bitlines are coupled to sense amplifiers at opposite ends thereof and the bitlines are divided into sub-bitlines.
 41. A dynamic random access memory as claimed in claim 40 wherein each bitline is divided into two sub-bitlines. 